phone stuck on one corner when trying to open

Here is a guide to step you through the teardown if you're not already using it _{iPhone 3GS Teardown} , You don't say which corner but I'll guess it's one at the bottom by the home button, and I'll also guess you have both screw's removed that hold the screen assembly in, so if I'm guessing right, when I have the same problem, I use a plastic pry tool and at the same time I am using a suction cup to pull the screen away from the frame, I hold pressure with the tool on the chrome bezel while I pry up with the suction cup, sometimes you have a tug of war going on to pull it apart, eat your Wheaties, good luck !